1. Field of the Invention
The invention refers to an apparatus for measuring the surface hardness of building materials, as e.g. of concrete. The apparatus is provided with a test hammer which by spring force is driven and brought to impact against an impact pin resting on the surface to be measured. Furthermore, a measuring device is provided for determining the rebound height of the test hammer after said impact. The measured rebound height then is a measure for the hardness of the respective building material which is not destroyed by this measuring process.
2. Description of the Prior Art
An apparatus of this type is known as Schmidthammer and is widely used today. Its basic principle is disclosed in Swiss Patent specification No. 283 099. As already mentioned, the known apparatus comprises a test hammer, which is driven by spring force to impact against an impact pin, which is placed on the surface of a concrete element to be measured. By the force of the impact, the pin which has a spherically shaped tip slightly penetrates the concrete surface, whereby part of the impact energy is dissipated. The remaining energy then causes the hammer to jump back or rebound against the force of the spring. The rebound height of the hammer as a percentage of its forward path (i.e. the path from the beginning of the spring force action to the impact on the impact pin) is designated as a rebound value R. This value R is characteristic for the elastic and plastic behavior of the concrete near its surface.
From this R-value the compression strength of the concrete can be derived under certain conditions. This is done by means of empirical calibrations curves, in which each R-value is related to a corresponding compression strength value.
In the known apparatus called Schmidthammer the rebound height is measured on purely mechanical basis by means of a drag pointer which is taken along by the rebounding hammer.
The upper end position of the drag pointer then indicates the rebound height, which can be read on a scale arranged at the housing of the apparatus. The drag pointer is guided on a linear guide and must have a defined friction when displaced along this guide. The friction value has to be in the range between 50 g and 80 g to prevent any displacement of the drag pointer above the real rebound height on the one hand and to avoid excessive braking of the test hammer by the drag pointer on the other hand. Especially because of the fact that the pointer guide gets dirty during use, the friction of the drag pointer has often to be measured and readjusted. A further drawback of the known apparatus results from the relatively coarse scale which for reasons of readability can not be made very fine. The reading and recording of the measured values has to be done immediately after the test stroke while the apparatus remains placed on the concrete surface in order to avoid any displacement of the draw pointer by shaking the apparatus. This is at least unpractical and in certain positions of the apparatus even difficult. An automatic processing of the measured values is not possible.